[libc++] Remove non-standard member type iterator_type from __wrap_iter (#186871)

Resolves #186801
Removed the non-standard member type `iterator_type` from `__wrap_iter`.
This member exposed the underlying iterator type, and its removal
prevents users from relying on the implementation detail.

NOKEYCHECK=True
GitOrigin-RevId: feae3bc202cedbc1e83201eb8ea147f1a7729cd3